[BUG] Hide Duo Seat Utilization and Duo Availability Sections in SaaS Admin Instance Settings

What does this MR do and why?

Hides Duo seat utilization and Duo availability from the SaaS admin instance menu where they are currently visible (and broken). They shouldn't be there at all.

Related issue: #559002 (closed)

Changes in this MR:

• Hide Duo seat utilization and Duo availability in SaaS admin instance settings. A new prop isAdminInstanceDuoHome is added to help with determining when in admin settings (in SaaS GitlabDuoHome is shared between admin and group settings)
• Replace usage of hard-coded showDuoWorkflowSettings prop with isAdminInstanceDuoHome (for a more meaningful understanding of when to show workflow settings)
• Refactors gitlab_duo_home_spec.js for greater readability

References

Screenshots or screen recordings

SaaS admin instance Duo home page

Before	After
BrokenSaasDuoAdmin

Self-managed admin instance Duo home page (NO CHANGES)

Before	After

SaaS Duo group settings (NO CHANGES)

Before	After

Self-managed Duo group settings (NO CHANGES)

Before	After

How to set up and validate locally

Set-up

• An active online cloud, legacy or offline Ultimate license.

Validating SaaS settings

1. Start GDK in saas mode GITLAB_SIMULATE_SAAS=1 gdk start to validate saas settings
2. Navigate to the SaaS instance settings home page - http://gdk.test:3000/admin/gitlab_duo and ensure the Duo seat utilization and availability screen are not there (see "After" screenshot)
3. Run the Duo setup rake task GITLAB_SIMULATE_SAAS=1 bundle exec 'rake gitlab:duo:setup' to seed DB with a Duo test group (it's called gitlab-duo).
4. Navigate to the test group's Duo settings page http://gdk.test:3000/groups/gitlab-duo/-/settings/gitlab_duo. It should look like the "SaaS Duo group settings" screengrabs (no regressions)

Validating self-managed settings

1. Start GDK in self-managed mode GITLAB_SIMULATE_SAAS=0 gdk restart to validate self-managed settings
2. Navigate to the self-managed instance settings home page - http://gdk.test:3000/admin/gitlab_duo and ensure the Duo seat utilization and availability screen are there. It should look like the "Self-managed admin instance Duo home page screen" screengrabs (no refressions). Don't worry if you are missing the "GitLab Duo Self-Hosted" section, this requires seperate configuration and is unrelated.
3. Navigate to the Duo settings page of the test group set up previously - http://gdk.test:3000/groups/gitlab-duo/-/settings/gitlab_duo. It should look like the "Self-managed Duo group settings" screengrabs (no regressions)

MR acceptance checklist

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